1. Field of the Invention
The invention relates to an imaging device in a projection exposure machine for microlithography, having at least one optical element and at least one manipulator, having a linear drive, for manipulating the position of the optical element.
2. Description of the Related Art
In the case of such imaging devices for projection exposure machines in lithography, also denoted lithographic optics for short, it is often advantageous if individual optical elements can be positioned actively during adjustment and/or during operation, in order to set specific imaging properties and aberrations precisely. Thus, for example, in rotationally symmetrical imaging systems, adjusting optical elements while maintaining the rotational symmetry, for example in the case of rotationally symmetrical refractive objectives, permits the displacement of lenses in the light direction (defined in the direction of the z-axis), the influencing of the focus, of the reduction ratio, of the 3rd order distortion, of the field curvature, of linear coma and of the constant spherical aberration. Moreover, environmental influences of a rotationally symmetrical nature, for example a change in the atmospheric ambient pressure, in the internal pressure, in the atmospheric humidity and in the temperature including longitudinal temperature gradients as well as rotationally symmetrical components of the lens heating can be corrected, as is known from U.S. Pat. No. 4,961,001 and DE 37 33 823.
In the case of rotationally symmetrical imaging systems, adjusting optical elements in conjunction with cancellation of the rotational symmetry and production of monochromatic symmetry for example in the case of rotationally symmetrical refractive objectives, permits lenses to be displaced perpendicular to the z-axis, specifically in the x-y plane, also termed lens decentering, or the tilting of lenses about axes perpendicular to the light direction, and the exertion of influence on centering errors that are expressed in nonrotationally symmetrical aberration profiles of monochromatic overall symmetry with reference to pupil and field. Included therein are, for example, image offset, sagittal and tangential 2nd order distortion, linear image surface tilt and the constant coma. Furthermore, environmental influences of monochromatic symmetry, for example gradients of ambient pressure, internal pressure, air humidity and temperature perpendicular to the light direction, can be corrected.
Moreover, it is very advantageous in catadioptric lithographic optics with plane deflecting mirrors or beam splitter cubes to be able to manipulate the position and tilting angles of the deflecting mirror or beam splitter surfaces. For concave and convex reflecting surfaces in catadioptric or catoptric lithographic optics, it is also suitable to manipulate the degrees of freedom in translation and tilting, in order to be able to set rotationally symmetrical aberrations and centering errors precisely.
Such imaging devices with manipulators are known in this case from the prior art.
For example, details may be given here of such a manipulator with the aid of the design described in U.S. Pat. No. 5,822,133. The manipulator there is designed as a pure z-manipulator in the cases of application described. This means that the manipulation is performed in the direction of the optical axis normally denoted by “z”. The design comprises two annular elements which are arranged one in another and can be moved relative to one another by actuators over a range of embodiments. Provided for guiding the parts relative to one another are leaf springs or, in accordance with a further refinement, diaphragms that are intended to ensure parallel movement of the two parts relative to one another.
However, such a design has decisive disadvantages, depending on embodiment, in particular on the embodiment of the various actuators described. Thus, for example, when pneumatic actuators are used it is possible to achieve only a relatively low rigidity in design. When vibrations are correspondingly introduced, and when use is made of very heavy optical elements, for example the very heavy lenses that are used in microlithography or in astronomical applications, this low rigidity of the manipulator leads to severe disadvantages that have very negative effects on the imaging quality to be attained.
The use of hydraulic actuators is proposed in a further refinement. It is certainly possible to attain a far higher rigidity with such actuators than with the pneumatic actuators previously described. However, the hydraulic actuators harbor the risk of the components to be manipulated being contaminated by the hydraulic fluid, in general an oil, should there be a leak. Such a contaminaion with hydraulic fluid is to be regarded as a severe disadvantage particularly in the case of high-performance objectives such as are used, for example, in microlithography. Such objectives are usually filled with a defined gas mixture or else are evacuated, if appropriate. Should hydraulic fluid, in particular oil, pass into this ultraclean interior, this can pass as liquid or vapor into the region of the optical elements and be deposited on their surface. The imaging quality would then be severely degraded. The outlay on any possible cleaning would be extremely high.
Furthermore, appropriate actuators made from piezoelectric elements and lever gears are described in an alternative embodiment of the above-named document. In this case, the levers of the gears can be interconnected particularly via solid joints. This type of actuators can avoid the two above-named disadvantages. In this case, very good resolutions can be achieved with these actuators. However, these actuators have the severe disadvantage that they permit only a very small travel. Depending on application, in particular in the use, already mentioned repeatedly, in an imaging device for microlithography, the requirement for very good resolution is, however, mostly additionally accompanied by the requirement for a very large travel in relation to the possible resolution. These requirements, which often cannot be avoided for the purpose of attaining a very good imaging quality, cannot be attained by the design described in the above-named US document, and so said design disadvantageously fails to permit the desired imaging quality.
Further manipulators, which have, however, the same or very similar disadvantages, are described, for example, by DE 199 10 947 A1. This document exhibits a design in which movement of the optical element along the optical axis is achieved via actuators, for example piezoelectric elements, and a corresponding gear made from levers connected via solid joints.
A device for manipulating an optical element in a plane perpendicular to the optical axis is specified, for example, by DE 199 10 295 A1. There is a need in this case for at least two actuators that, because of the movement accuracies to be attained with these actuators, act via expensive and complex lever devices on an inner ring of the mount that carries the optical element. In order to ensure a uniform and adequate rigidity of the lever devices, and thus of the joining of the two parts of the mount to one another, as well to ensure an adequate resolution of the movement, there is a substantial outlay with respect to production, in particular with respect to the observance of very narrow manufacturing tolerances.
Furthermore, JP 3064372 discloses a device for manipulating optical elements in the case of which a first group of optical elements and a second group of optical elements are arranged such that they can be displaced along an optical axis by a manipulator. These two optical groups can also execute a tilting movement relative to the optical axis with the aid of the manipulator device. Electrostrictive or magnetostrictive elements can be used for the drives. The design of the rotary actuators is not disclosed directly in an unambiguous fashion in this document.
Reference may also be made regarding the further prior art to U.S. Pat. No. 6,150,750, which exhibits a linear drive for the field of electrical engineering, telecommunication engineering and automation. The linear drive has a driven subregion and a nondriven subregion that can be moved relative to one another in the direction of a movement axis, the subregions being interconnected at least temporarily via piezoelectric elements that are designed in part as lifting piezoelectric elements and in part as shearing piezoelectric elements. The individual piezoelectric element stacks, which comprise lifting and shearing piezoelectric elements in each case, can be brought by means of the lifting piezoelectric elements into frictional grip relative to the driven subregion, or can be raised thereby. The actual movement can then be realized via the shearing piezoelectric elements of those stacks that are in frictional engagement. It is possible thereafter to conceive of moving on to other stacks, and so very large movement ranges can be realized.
This design of the linear drives results in practice in an actuator that is capable, via its piezoelectric elements, of exerting holding forces and forces in the direction of the movement axis on the two parts of the linear drive. It is thereby possible to implement a linear stepper that, depending on the selection of the piezoelectric elements, has a very good resolution and, on the basis of the possibility of moving on, also has a very large allowance for movement.